# C++ named requirements: LegacyOutputIterator

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A LegacyOutputIterator is a LegacyIterator that can write to the pointed-to element.

An example of a type that implements LegacyOutputIterator is std::ostream_iterator.

When LegacyForwardIterator, LegacyBidirectionalIterator, or LegacyRandomAccessIterator satisfies the LegacyOutputIterator requirements in addition to its own requirements, it is described as mutable.

### Requirements

The type X satisfies LegacyOutputIterator if

- The type X satisfies LegacyIterator
- X is a class type or a pointer type

And, given

- o, a value of some type that is writable to the output iterator (there may be multiple types that are writable, e.g. if operator= may be a template. There is no notion of `value_type` as for the input iterators)
- r, an lvalue of type X,

The following expressions must be valid and have their specified effects

Expression | Return | Equivalent expression | Pre-condition | Post-conditions | Notes
--- | --- | --- | --- | --- | ---
*r = o | (not used) |  | r is dereferenceable | r is incrementable | After this operation r is not required to be dereferenceable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable.
++r | X& |  | r is incrementable | r and ++r designate the same iterator object, r is dereferenceable or past-the-end | After this operation r is not required to be incrementable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable.
r++ | convertible to const X& | X temp = r; ++r; return temp; |  |  |
*r++ = o | (not used) | *r = o; ++r; |  |  |

### Notes

The only valid use of operator* with an output iterator is on the left of an assignment: operator* may return a proxy object, which defines a member operator= (which may be a template).

Equality and inequality may not be defined for output iterators. Even if an operator== is defined, x == y need not imply ++x == ++y.

Assignment through the same value of an output iterator happens only once: algorithms on output iterators must be single-pass algorithms.

Assignment through an output iterator is expected to alternate with incrementing. Double-increment is undefined behavior (C++ standard currently claims that double increment is supported, contrary to the STL documentation; this is LWG issue 2035).

Pure output-only iterator is allowed to declare its iterator_traits<X>::value_type, iterator_traits<X>::difference_type, iterator_traits<X>::pointer, and iterator_traits<X>::reference to be void (and iterators such as std::back_insert_iterator do just that except for `difference_type`, which is now defined to satisfy std::output_iterator(since C++20)).

### Standard library

The following standard library iterators are output iterators that are not forward iterators:

ostream_iterator | output iterator that writes to std::basic_ostream (class template)
ostreambuf_iterator | output iterator that writes to std::basic_streambuf (class template)
insert_iterator | iterator adaptor for insertion into a container (class template)
back_insert_iterator | iterator adaptor for insertion at the end of a container (class template)
front_insert_iterator | iterator adaptor for insertion at the front of a container (class template)

### See also

output_iterator (C++20) | specifies that a type is an output iterator for a given value type, that is, values of that type can be written to it and it can be both pre- and post-incremented (concept)
Iterator library | provides definitions for iterators, iterator traits, adaptors, and utility functions

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